1. Field of the Invention
The invention concerns a measuring and/or security system of the type given in the precharacterizing clause of Claim 1.
2. Description of Related Art
Measuring and security systems are needed today in a multiplicity of types for the most varied of applications. One of these is the protection of objects against theft.
A system of this type is known, for example, from the German published patent application DE 36 18 416 A1, and consists essentially of two transmitting/receiving devices, one of which is installed on the object to be protected, while the other is arranged in fixed fashion in a monitoring unit. During operation, the two transmitting/receiving devices communicate with each other, with the one transmitting/receiving device generating a control signal, to which the other transmitting/receiving device answers with an acknowledgment signal. After receipt of the acknowledgment signal, the transmitting/receiving device that radiates the control signal measures the time difference between the sending out of the control signal and the receipt of the acknowledgment signal, and upon exceeding a prescribed signal transit time triggers an alarm.
In the case of the previously-known measuring and/or security system, however, it is disadvantageous that the transmitting power required for a certain transmission of data between the two transmitting/receiving devices is relatively high.
For one thing, this limits the operating time of the security system, since the transmitting/receiving device installed on the object to be protected is battery operated. For another thing, because of the relatively high transmitting power, it can result in interference with other communication systems, in particular other security systems.
Therefore, the task underlying the invention is to obtain a measuring and/or security system of the initially-mentioned type having a reduced transmitting power.
This task, starting out from a measuring and/or security system based on the pre-characterizing clause of Claim 1, is resolved by the characterizing features of Claim 1.
The invention includes the technical teaching of generating on the transmitter side, as control and/or acknowledgment signals, pulses that shift in frequency over the duration time of the pulse, and of compressing these pulses on the receiver side by means of dispersion filters.
A dispersion filter of this type is, in principle, actually known from the European patent application EP 0 223 554 A2, without more detailed application instructions having been given.
The invention starts out from the knowledge that the pulses transmitted as control and/or acknowledgment signals, for achieving a high precision when measuring distance, may display only a slight temporal imprecision. On the other hand, in order to be able to transmit these types of short pulses, a relatively wide bandwidth is required.
With the invention a measuring and/or security system for measuring distance is created that is based on a measurement of the signal transit time, and that, even with a low transmitting power in signal-interference surroundings, nevertheless can operate with a minimum temporal imprecision of the emitted signal.
In the case of the measuring and/or security system in accordance with the invention, chirp signals are radiated for determination of transit time, said chirp signals being compressed in time in the receiver by a suitable dispersion filter, in order to achieve a low temporal imprecision as a presupposition for an exact distance measurement, and furthermore to raise the amplitude on the receiver side.
In addition to this, at least one of the transmission systems displays on the transmitting side a signal source that generates, as a measurement signal, pulsesxe2x80x94also designated as chirp signalsxe2x80x94having a frequency that falls or rises monotonically during the duration of the pulse and that corresponds to a prescribed, preferably linear, modulation characteristic curve. However, it is not necessary that the modulation characteristic curve be linear. It is critical merely that the frequency, in each instance, rise or fall monotonically, in the mathematical sense, over the duration of the pulse.
On the other hand, the receiving transmission system displays for pulse compression a dispersion filter having a frequency-dependent signal transit time corresponding to a prescribed filter characteristic curve. It is important here that the filter characteristic curve be matched to the proposed modulation on the transmitter side such that the signal portions of the modulated pulses corresponding to this modulation characteristic curve, because of the different frequency-dependent signal transit time through the dispersion filter, appear at its output essentially coincidentally, and subsequently with lesser temporal imprecision.
In one variant of the invention, as an approximation, a Dirac pulse is first generated and fed to a low-pass filter whose filter characteristic displays a peaking shortly before reaching the limit frequency, and therewith transforms the Dirac pulse into a sinc pulse, whose form is described by the known sinc function sinc (x)=sin x/x. The sinc-form output signal of the low-pass filter is next passed over to an amplitude modulator, which impresses on the carrier oscillation a sinc-form envelope. If the signal generated in this fashion is fed to a dispersion filter, then appearing at the output is a frequency modulated pulse. In this variant of the invention, therefore, there first follows on the transmitter side an expansion of the relatively sharp sinc-pulse through the dispersion filter into a frequency modulated pulse, which in comparison to the sinc-pulse is lengthened, and displays a correspondingly lower amplitude.
According to another variant of the invention, generation of the frequency modulated pulse, on the other hand, is accomplished by means of a PLL-loop (PLL: Phase Locked Loop) and a Voltage Controlled Oscillator (VCO). For this purpose, a square pulse is first converted into a sawtooth shaped pulse. The pulse generated in this fashion is then used for control of the VCO, such that the frequency of an output pulse increases or decreases linearly during the duration of the pulse.
In another variant of the invention, generation of the frequency modulated pulse is accomplished in the digital signal processing unit, which advantageously enables realization of random modulation characteristic curves.
The previously mentioned dispersion filters are preferably realized as Surface Acoustic Wave (SAW) filters, since these types of filters can be produced with a high degree of precision and stability. Moreover, these types of surface acoustic wave filters offer the advantage that amplitude response and phase response can be dimensioned independently of one another, which opens up the possibility of realizing, in a single structural element, the narrow band bandpass filter and the dispersion filter required in each receiver.
According to the invention, distance measurement between two previously described transmission systems is accomplished with one transmission system preferably being installed on or in the object of measurement, and with the other transmission system preferably being arranged in fixed fashion.
For distance measurement, the first transmission system generates, by means of a transmitter, a measuring signal that is radiated to the second transmission system and there detected by a receiver. Measurement of the signal transit time is accomplished by a time measuring device that is connected on the input side with the receiver of the second transmission system.
Interval determination is accomplished by determining the time duration of signal travel, to and return, between two transmitter and receiver pairs. Distance measurement is introduced herewith by the transmitter of the second transmission system emitting a measuring signalxe2x80x94also designated in the following as a control signalxe2x80x94to the receiver of the first transmission system. Upon receipt of this measuring signal, the receiver of this transmission system triggers the transmitter of the same transmission system, which, thereupon, emits a measuring signal as an acknowledgment signal, and transmits it to the receiver of the second transmission system.
The measured signal transit time here corresponds, therefore, to double the distance between the two transmission systems.
In the previously described variant of the invention, distance measurement between the two communicating transmission systems is accomplished by measuring the signal transit time between emission of the control signal by the one transmission system and the subsequent receipt of the outgoing acknowledgment signal from the other transmission system. In one advantageous variant of the invention, for increasing measurement accuracy provision is made for determining the internal signal transit times, and for taking into account in the calculation the distance between the communicating transmission systems.
In addition to this, the control signal generated on the transmitter side, without the detour over the other transmission system, is passed on directly to the receiver of the same transmission system. For example, it is additionally possible to use the same antenna for emission of the control signal and for reception of the acknowledgment signal, so that the control signal emitted on the transmitter side also directly reaches the receiver side of the same transmission system. Instead of this, however, it is also possible in normal operation to separate the transmitter and receiver of the individual transmission systems and to connect them together only in a time-limited fashion during the calibration procedure. Since in the case of direct control of the receiver the control signal is essentially stronger than the acknowledgment signal, in this variant of the invention it is advantageous to temporarily adjust the sensitivity of the receiver downward immediately after emission of the control signal so that the control signal does not over-control the receiver. After receiver side detection of the control signal emitted by the same transmission system, the time measurement device determines the internal signal transit time between the emission of the control signal and detection of the same control signal, which is then intermediately stored in a memory unit. Consideration of the internal signal transit time is then accomplished through a calculating unit that computes the distance between the two communicating transmission systems from the intermediately stored internal signal transit time and the total transit time determined by the time measuring device upon arrival of the acknowledgment signal. Since at any given time only the internal signal transit time in one transmission system is determined in the above-described manner, while the distance measurement is also falsified by the internal signal transit time in the other transmission system, the calculating unit preferably subtracts from the measured total transit time double the value of the internal signal transit time determined for one transmission system. It is also possible to determine the internal signal transit time in both transmission systems and to transmit it at any given time to the other transmission system, for example, together with the control and/or acknowledgment signal. In this case, the calculating unit subtracts from the total transit time the internal signal transit times determined for both transmission systems, which in the case of differently constructed transmission systems and consequently also of different internal signal transit times, brings about an improvement in measuring precision.
If other information-carrying signals besides the control and/or acknowledgment signal are transmitted between the two transmission systems, as for example the communication of the internal signal transit time in the previously described variant of the invention, this communication is then preferably accomplished with an error-correction code that enables recognition of a transmission error. The control and acknowledgment signal, by means of detection of corresponding recognition signals, enables a mechanism that ensures that only the signals of transmitter/receiver systems associated with one another trigger a measuring process. Therefore, through the use of chirp signals in accordance with the invention, it is possible for several like type systems to operate in coincident frequency bands next to each other, without interfering with each other.
In another advantageous variant of the invention, the measuring and/or security system serves for detection of the removal of an object from a predetermined area or for detection of penetration into the area of the object, in particular for protection against theft in a warehouse. As a rule, for this purpose a transmission system is installed on a movable object to protect it against unauthorized removal, with the supply of energy for this transmission system as a rule being provided by a built-in battery, so that maximum operation time of the security system is limited by the useful life of the battery.
In an improved variant of the invention, therefore, provision is made for reducing power consumption in order to increase the useful life of the battery. For this purpose, the transmitting power or the input amplification is matched to the transmission conditions prevailing at the place of use. Thus, in the case of weak interfering signals only low transmitting power and/or low input amplification is required, whereas, on the other hand, in strong interference surroundings, a correspondingly greater transmitting power and/or input amplification is needed. Therefore, the transmission system in this variant of the invention displays on the receiver side a field strength measuring system that, for one thing, measures the field strength of the signals sent from the other transmitter/receiver device and, for another thing, measures the noise in order to determine from this, with a calculator, the signal-to-noise ratio. The value determined in this way then serves for controlling a regulator that is connected at the output side to an input amplifier arranged on the receiver side for regulating the input amplification, or is connected to the transmitter that generates the control signal for regulating the transmitting power. Regulation of input amplification and/or transmitting power is accomplished such that the signal-to-noise ratio, on the one hand, does not fall below a minimum value required for normal transmission between the transmission systems, however, on the other hand, transmitting power is held as low as possible in order to not needlessly consume energy.
According to an advantageous development of the invention, the transmission systems of the electronic security system communicating with one another display control means for the purpose of carrying out the communication function of the security system transmission system in the case of intermittent operational management. This measure enables a further reduction of energy consumption and therewith an increased battery life.
With intermittent operation of the security system, transmitters and receivers of the communicating transmission systems work in the above-described way and manner, at any given time, for example, only 500 xcexcs with a cycle time of one to two seconds.
This means that for a very long (electronic) time of about half a second they do not operate and, thus, for the battery delivering the supply voltage, an on/off duty cycle favorable for conserving energy can be realized.
Furthermore, the invention includes the teaching of encoding the control signal and/or the acknowledgment signal, in order to prevent an imitation of the acknowledgment signal for falsification purposes, for example by a thief.
In doing this it is particularly beneficial to not concentrate on increasing the complexity of the code, with correspondingly high expense for decoding, but rather to provide in advantageous fashion means through which a code that is used can be changed in a prescribed way.
According to a preferred embodiment of the invention, the anti-theft protection system displays two wireless transmission systems alternately communicating with one another, in which is provided in each case a transmitter and a receiver. The transmission of information advantageously occurs through high frequency-, light- or acoustic energy. The transmission systems communicating with one another are constructed essentially identically and in each case display primary electronic control means, such that the transmitting arrangement of the first transmission system can emit a coded control signal, which, after receipt by the second transmission system stimulates the latter to emit a coded acknowledgment signal intended for receipt by the first transmission system. Here, the coding of the acknowledgment signal is different from that of the control signal. After receipt of the acknowledgment signal in the first transmission system, this latter again emits a control signal that displays a code differing from the code of the last signal. In this way there results a continuous, alternating connection between the two communicating transmission systems of the security system in accordance with the invention. The code is changed in random fashion, with the speed of the alternating transmission being so great that, even with computers of highest computing power, no decoding could result within the available time. If decoding does result later, the code has already changed several times.
For increasing alarm security and for preventing disturbing interference by other similar systems located close by, the primary electronic control means are built into the transmission systems communicating with each other, such that the coding of control signal and acknowledgment signal is changed after each transmitting and receiving cycle.
The primary electronic control means display a code manager combined with a random number generator and a memory unit that, based on a distinct principle in accordance with the invention, controls the cyclically performed changing of the coding of the transmitted signals. The signal to be emitted from the respective transmitting arrangement is first generated by the random number generator and then encoded. The code Knxe2x88x921 is formed from an already-sent signal in order, in this manner, to receive as a key an actually statistically varying code, dependent, however, on the prior operational management of the two communicating transmission systems. This code is stored in recallable fashion in the memory unit. The signal to be sent is encoded based on the preceding code Knxe2x88x921, and transmitted. At the same time, the transmitter of the one transmission system forms from the actual random number and the code Knxe2x88x921 a new code Kn, and stores it in the memory.
The signal originating from the transmitter is decoded and checked for compatibility by the other transmission system via the applicable key Knxe2x88x921 that is known from the previous transmission. The result is recognized in this transmission system, stored, and drawn upon for forming a new code Kn, which the transmitter of the other transmission system had already formed before emission of the signal. The transmitter of the transmission system generates a new signal via the appropriate random number generator, codes it according to Kn and sends it to the first transmission system. Simultaneously formed and stored thereby is the code Kn+1. The receiver of the now receiving transmission system can decode the signal sent from the other transmission system, since it itself has formed the code Kn.
Since the transmitters and the receivers of the two transmission systems communicating with each other have, since the first initialization, regularly started out from the same signal basis, and the statistically generated signals are known through the preceding signal dialog, of pressing importance, in an advantageous way, in the security system is a code constantly changing in a selectably-adjustable cycle. This code, because of the correlation at any given time with the previous signal dialog, is known, however, only to the two transmission systems communicating with each other, and can also be checked only by them for compatibility.
Available by this means is a measuring and/or security system that offers, in an advantageous manner, an extremely high security factor for its operating management, and which cannot be compromised by intruders, because of the continually changing code and the indeterminable operating cycle.
The above-described security system is also suitable in this form as a warning system for forgotten objects, for example money purses and appointment books in telephone booths, or as a security system for adults who would wish to maintain control of their children who are to be watched over in a larger group of people.
In accordance with the preferred form of embodiment of the invention, the transmission systems of the security system communicating with each other are constructed in the form of a chip card or as a small-format utility object, for example as a cigarette lighter. In this way, they are capable of being produced economically as a mass-production article, and, among other things, offer a further field of application in combination with a bank card, credit card or identity card.
A multi-layer construction is beneficial in the manufacture of the transmission systems of the security system as mass production articles in the form of chip cards, in order to take into account space expansion for the power supply sources, audible alarm systems and/or means for programming the primary electronic control means.
In accordance with another advantageous development of the invention, to expand the area of action of the measuring system in a simple way, several like-constructed transmission systems are positioned at control points, maximally at an interval between each other of the greatest coverage of the transmitter used. The object (objects) to be protected is (are), in each case, to be provided with a transmission system. In the case of unauthorized departure from the permitted area, at least one of the transmission systems reacts by triggering an alarm.
In the case of an expanded activity area of the measuring system, in order to prevent the dialog of the transmission systems connected to each other at any given time from being disturbed, for example by an overlapping frequency or interference phenomena, there occurs through the second electronic means an automatic shifting of the dialog time window for the respective pair. This is possible without problem, since, with a cycle time of preferably 1 sec. for the random number generator and a transmitting time of about 500 xcexcs in a so-called staggered operation, more than one thousand time windows can be realized. The shifting of the time window used for transmission of the control signal and the acknowledgment signal is advantageously accomplished here randomly, in order to prevent the two pairs of transmission systems from shifting their transmission window together, which, in spite of the present shift, would lead to a constant overlapping and mutual interference.
Even in the case of this type of random shifting of the time window provided for transmission of control and/or acknowledgment signal, it is, however, possible that the time windows of two pairs of transmission systems briefly coincide. In this case, transmission between the transmission systems would be disturbed, such that the exchange of control and acknowledgment signal would not function. Therefore, in the preferred form of embodiment of this variant provision is made to trigger the alarm only when the acknowledgment signal also fails to appear for a predetermined number of control signals, so that short transmission disturbances will not lead to triggering an alarm. For this purpose, the transmission systems display a storage element in which are stored the number of already-omitted acknowledgment signals.
With omission of an acknowledgment signal, the content of this storage element is incremented, where on the other hand, the storage element is reset upon appearance of an acknowledgment signal. Moreover, the transmission systems in this variant of the invention display a comparison unit that compares the content of the storage element with a predetermined limit value, and upon exceeding the limit value triggers an alarm.
In another variant of the invention, provision is made on the transmitter side to generate different angle-modulated pulses, and at the receiver end to compress these by means of appropriately matched dispersion filters. If the information-carrying signal is present in binary form as a bit sequence, then, in the case of a logical LOW-level, it is possible to transmit a linearly-increasing frequency during the duration time of the pulse, where on the other hand, in the case of a logical HIGH-level, a pulse having a linearly-decreasing frequency is transmitted during the duration time of the pulse. In this case on the receiver side two dispersion filters are required with appropriately matched, frequency-dependent transit time characteristics.
By transmission of differently angle-modulated pulses, it is possible, for one thing, to actively transmit both logic LOW-levels as well as HIGH-levels, which contributes to increasing protection against interference. For another thing, the possibility here arises of transmitting a greater amount of data with each transmission pulse. For example, if eight different pulses are available for the transmission, it is then possible to transmit with each pulse an amount of data of log28=3 bits.